During the metalworking process metal is bent, formed, perforated, cut or stamped to produce a desired final or intermediary product. During each of the above processes, the blank, which is the material being worked, tends to move toward the site of the particular operation. This movement is caused by the bending and stretching of metal at the site of the operation as a result of the application of force to the blank. It is desirable in many situations that such movement be controlled and/or minimized, if not entirely eliminated, by holding the blank in a fixed position relative to the relevant die.
In order to releasably secure a blank in place, force must be exerted sufficient to overcome the lateral force applied by the impact of the die upon the blank. This is usually accomplished by the application of a clamping force roughly perpendicular to the plane of the lateral force. The clamping force must be sufficient to produce a lateral frictional resistive force sufficient to overcome the lateral component of the force being applied by the die.
Due in part to the need to quickly and smoothly apply such a force, it is common to utilize springs of one nature or another to attempt to hold the blank in place. Gas springs, rubber blocks and coil springs have been variously applied for this purpose but have been found wanting. Their chief disadvantages are the limitation on the maximum resistive force that can be generated by any such device and the necessity for frequent service. Typically, gas springs are initially pressurized to about 2500 p.s.i. The force produced by such a gas spring is only 2500 p.s.i. times the area of the piston rod chosen. Typically, space or room in a die is at a premium, as one skilled in the art would be aware, and any method to reduce the volume occupied by a clamping spring is a significant advantage. The known devices have the disadvantage of typically not being capable of producing enough force to fully and properly clamp the blank while at the same time taking up valuable room in the die. The use of the liquid spring described herein provides the advantage of generating a very high clamping force while occupying a space equal to or less than the known spring devices.
The use of springs of various kinds in the dampening of movement of dies and other metalworking equipment is also known. Gas springs have been used in a variety of die assemblies to apply yieldable resisting, biasing or cushioning forces to dies, as taught for example in U.S. Pat. No. 3,947,005, issued Mar. 30, 1976 to Wallis. The Wallis device incorporates a plurality of cushioning gas springs in a die assembly, which are utilized to apply a yieldable resisting force to a pressure pad used in a stamping press. The Wallis device also utilizes a gas spring in an ejector mechanism designed to remove the stamped work piece from the die member once complete. The Wallis springs are pressurized with an inert gas such as nitrogen, which can be supplied from a fluid accumulator via flexible conduits.
The use of a liquid filled device to eject punched material from a blank is described in U.S. Pat. No. 2,704,125, issued Mar. 15, 1955 to Taylor. The cylinder/plunger combination of this device is located centrally within the punching die in order to propel the workpiece out of the die assembly once cut free of the blank.
U.S. Pat. No. 3,180,634, issued Apr. 27, 1965 to Heiser, teaches the use of a liquid spring to simultaneously cushion die members and provide a biasing force to urge a pressure pad against the blank. The spring in the Heiser reference is a liquid spring incorporating a piston and cylinder arrangement wherein the piston is closely fitted to the inside diameter of the cylinder vessel. The volume of liquid in the cylinder is pressurized by an outside hydraulic system and therefore the spring is not a self-contained closed system as taught herein. Such an arrangement is undesirable because the repeated impact shock of the descending die will interfere with the integrity of hydraulic fittings in high pressure hydraulic systems and such a system requires that hydraulic hoses, fittings and other components be connected up to the press and therefore would be required to move constantly with the press movement. It is also difficult with such a hydraulic system to generate sufficient clamping and reactive force from the spring. Also, because of the complexity of the die, it can be very difficult to feed it from a hydraulic pressure source. The Heiser device does not rely on any compressibility of its liquid in its operation, using only a standard hydraulic fluid, and is thus unable to take advantage of the high levels of reactive force that may be achieved with compressible liquids.
Other liquid springs are known which utilize compressible liquids in different applications other than metalworking. The compressibility of liquids, and therefore the reactive force capable of being generated by springs employing compressible liquids, increases dramatically with increased initial pressurization. It has not hitherto been appreciated that it is possible to use, obtain or build a self-contained, pre-pressurized liquid spring which is capable of delivering substantially increased clamping forces over conventional gas springs in order to hold a blank in place.
For example, a gas spring is not generally pressurized over 2500 psi. A pressure of 2,500 p.s.i. is often the highest used in gas springs because this is the pressure typically available from gas supply bottles or containers. A liquid spring can be pre-pressurized, if needed, to a level of 20,000 psi or higher depending on the desired characteristics. Thus, an area of one square inch in the gas spring of this example will produce a reaction force of 2500 lbs. while a liquid spring can produce an initial reaction force of 20,000 lbs or eight times more force.
An object of the invention is to provide a metal working press with one or more liquid springs adapted for clamping a metalworking blank or workpiece in place during the metalworking process, these springs employing a compressible, pressurized liquid.
It is a further object of the invention to provide a metal working press with at least one improved liquid spring which is capable of producing significantly higher reactive force in response to a given compressive force than that previously available with gas springs of conventional design.
The terms "blank" and "workpiece" are used interchangeably herein to refer to the unfinished workpiece or item being processed in the metalworking press. As used herein, the term "workpiece" is a broad term that includes a blank which generally refers to flat pieces of metal that have not been bent or worked by a press previously.